This application is a continuation of U.S. application Ser. No. 136,600 which is a continuation of U.S. application Ser. No. 896,360, now U.S. Pat. No. 4,728,499.
It is understood that exhaled respiratory gas contains a substantially higher concentration of carbon dioxide, usually on the order of 3% to 5%, than does ambient air which normally contains about 0.03% carbon dioxide. Accordingly, the detection of a sufficient concentration of carbon dioxide in a gaseous sample can be considered evidence of respiratory gas.
Numerous techniques and devices which detect the presence of carbon dioxide in a gaseous sample have been disclosed or suggested heretofore. One such technique involves the detection of carbon dioxide through the use of certain chemical compounds which change color according to the pH of their environment. The utility of such chromogenic pH-sensitive indicators in connection with the detection of carbon dioxide is well understood. For example, U.S. Pat. No. 2,890,177 discloses a highly sensitive liquid chemical indicator capable of detecting the presence of carbon dioxide in the range of about 1/2% concentration or less. U.S. Pat. No. 3,068,073 discloses a method of detecting the relatively low concentration of carbon dioxide found in air. The principal utility disclosed is for field or plant use and neither of these references addresses the problems of detecting the presence of carbon dioxide when it is present in diagnostically significant concentrations, i.e., on the order of about 2% to about 5%.
U.S. Pat. No. 3,114,610 discloses a continuous sampling gas analyzer comprising a pH-sensitive dye suspended in a gel substance, a gas-permeable membrane, a light source for illuminating the dye and a detector for analyzing the light transmitted through the dye. U.S. Pat. No. 3,754,867 discloses a gas analyzer which also uses a light source and which transmits light through a multi-layered sensor unit. A detector analyzes the color change of a pH-sensitive indicator in one of the layers. The color change is apparently a function of the concentration of carbon dioxide in the gas being measured. In U.S. Pat. No. 2,136,236 to Draper complex and expensive equipment is disclosed for use in monitoring the carbon dioxide content of anaesthetic gas going to a patient by bubbling the gas through bulk liquids.
The various devices and compositions disclosed in the above-mentioned patents provide means for detecting or indicating the presence of carbon dioxide under certain circumstances. However, none of these references directly addresses the problem of determining accurately and rapidly the correct positioning of an endotracheal catheter or the detection of an apneic patient.
Introduction of a catheter in the trachea of a human may be required for a number of reasons. For example, in a hospital, an endotracheal catheter, may be used for general anesthesia; in the field, an endotracheal catheter may be needed to resuscitate an apneic patient. In both of these instances, and others, it is critical that the catheter be properly placed in the trachea and not, for example, in the esophagus. If the catheter is improperly placed and the error is not discovered within a very short time, on the order of less than about 20 seconds and preferably within about 2 to about 10 seconds, the patient may begin to suffer irreparable harm or even death.
In view of the criticality of rapid determination of when an endotrachael catheter is improperly placed, the need for a simple device which will rapidly and reliably give an indication of improper (or proper) placement is evident. One early attempt to simplify, expedite and make safe the technique of introduction of an intratracheal catheter is disclosed in U.S. Pat. No. 2,638,096. There, a perforate whistle is adapted to sound an audible signal in the presence of feeble breathing. Such a device has obvious drawbacks and never achieved substantial popularity in the medical field. A complete review of the problems inherent in esophageal intubation and various available techniques for properly locating the catheter is set forth in an article by P. K. Birmingham et al., "Esophageal Intubation: A Review of Detection Techniques." Anesth. Analg. 1986. 65, pp. 886-891.
Several efforts have been made heretofore to construct apparatus capable of detecting carbon dioxide in exhaled breath and thereby ensure proper location of an endotracheal catheter. One such technique, known as capnography, provides a continuous record of expired carbon dioxide, called a capnogram. Such techniques are disclosed by Z. Kalenda in Resuscitation 6, 259-263 1978. Capnography equipment is complex, expensive and often involves infrared spectroscopy.
Relatively less complicated techniques and devices have been also suggested for use in confirming intratrachel placement of a catheter. For example, Berman et al. in Anesthesiology Vol. 60, No. 6 June 1984 disclosed the use of a bulk liquid solution through which exhaled air is bubbled. The bulk liquid solution is contained within a DeLee mucus trap and consists of a mixture of 3 ml of phenophthalein and 3 ml cresol red. Respiratory carbon dioxide causes a color change in the solution. The device of Berman et al., however, is not easily transportable, functions properly only in one position and poses an inherent risk to the patient insofar as improper use might cause the solution to flow into the patient. U.S. Pat. No. 4,691,701 to Williams also purports to relate to determining the location of the endotracheal tube in a patient and the use of a particular type of carbon dioxide detector. However, Williams fails to disclose the capability of or any basis for distinguishing within a relatively rapid period of time between respiratory gas and, for example, ambient air.